This is deranged. Putin presides over a weak country with a declining native population. Losing several European (let alone American) major cities with millions of people dead to avoid "appeasing him" would be a far greater cost than the gain, particularly from a national perspective.
If we appease in Ukraine, we will have shown that Ukraine made a horrible mistake by giving up their nukes and signing the Budapest Memorandum, and other countries will be sure not to repeat Ukraine's mistake.
If we let Vietnam fall to the Communists, we will have shown the Communists we are weak and the rest of Asia will fall.